1. Field of the Invention
The present invention relates to a microwave system and other high-frequency systems and, more particularly, to a connection structure for connecting a coaxial cable with a planar circuit substrate of a high-frequency system.
2. Description of the Related Art
Recently, various apparatuses including a high-frequency circuit, such as mobile communication devices, have been used. In most cases, a high-frequency circuit using a high frequency of no less than 5 MHz, such as a microwave frequency, is mounted on a planar printed circuit board.
A coaxial cable is a medium capable of transmitting both an analog signal and a digital signal and has the structure shown in FIG. 1. In FIG. 1, in a coaxial cable 10, an internal conductor 2 is covered with an internal insulator 4 formed of dielectric material. The internal insulator 4 is covered with an external conductor 6, and the external conductor 6 is covered with an external insulator 8. Since the coaxial cable 10 is not considerably affected by external electromagnetic fields and has a low power loss, the coaxial cable 10 generally has been used for such high-frequency systems.
The coaxial cable 10 transmits high-frequency signals, inputted from a tester and other types of systems to a high-frequency device mounted on a planar printed circuit board of a high-frequency system. While the coaxial cable 10 provides a radially symmetric transmission structure for the signals, the transmission structure within the high-frequency device of the high frequency system is inherently planar. In addition, in a case where the signals are within a microwave or millimeter wave frequency range, discontinuities at an interface caused by the difference between the radially symmetric transmission structure and the planar transmission structure cause impedance mismatches that degrade the performance of systems.
In addition, a connector is generally required to mount the coaxial cable 10 having the radially symmetric transmission structure on a printed circuit board having the planar transmission structure.
FIG. 2 is a cross-sectional view illustrating a planar circuit substrate coupled with a coaxial cable using a connector disclosed in U.S. Pat. No. 5,797,765.
In FIG. 2, a planar substrate 12 has a conductive hole 16 and a contact pad 14 which is formed at the surface of the planar substrate 12. A connector 18 includes a conductive disk 22 which has a hole H and an edge 20 of an oblique angle, a protrusion 24 concentrically arranged in the hole 16, a cylinder-shaped sleeve 26 extending from the bottom surface of the conductive disk 22, and an internal conductor 28 positioned along the axis of the cylinder-shaped sleeve 26. The protrusion 24 is positioned in the conductive hole 16, and the internal conductor 28 is positioned at the center of the conductive hole 16.
However, the use of the connector 18 increases the volume of a system and the cost of manufacturing the system. In addition, the problem of impedance mismatches caused by discontinuities at an interface is not considered in the disclosure of U.S. Pat. No. 5,797,765. Moreover, since U.S. Pat. No. 5,797,765 discloses that the internal conductor 28 is wire-bonded by a wire 23 to a strip line 15, it is not appropriate for mass production of the system.
To solve the above-described problems, it is an aspect of the present invention to provide a connection structure of a coaxial cable which is capable of easily connecting the coaxial cable to a planar circuit substrate without using a connector.
To further solve the above-described problems, it is an aspect of the present invention to provide a connection structure of a coaxial cable which is capable of improving the transmission performance of the coaxial cable by minimizing impedance mismatches caused by discontinuities at a structural interface.
An apparatus consistent with the present invention relates to a connection structure, in which an external conductor of a coaxial cable is electrically connected to a ground conductor installed at a surface of a planar circuit substrate and an internal conductor of the coaxial cable is connected to a micro strip line installed at the other surface of the planar circuit substrate, the internal conductor of the coaxial cable being welded to a conductive pattern of the micro strip line, and the conductive pattern of the micro strip line comprising a conductive welding portion welded to the internal conductor of the coaxial cable, a conductive landing portion extending from the conductive welding portion in a predetermined width, and an impedance converting portion whose width increases as it extends from the conductive landing portion.
According to the present invention, the connection structure includes a coaxial cable and a planar circuit substrate which are connected, for example, by welding, soldering, or conductive adhesive, without using additional connectors. Thus, the connection structure is not only cost-effective but is also suitable for mass production due to its simplicity.
In addition, the present invention minimizes impedance mismatches caused by discontinuities at a structural interface such as a connection of a cylinder-shaped coaxial cable and a planar circuit substrate, by equipping a micro strip line with an impedance converting portion which increases in width from one end to the others.
It is preferable that at least one via hole is formed in a predetermined pattern surrounding the conductive welding portion. According to the present invention, the via hole improves signal transmission abilities of the coaxial cable by guiding signals transmitted from the coaxial cable. In addition, it is preferable that at least two via holes are formed in a symmetrical pattern.
According to the present invention, a test shows that the via holes, which are arranged in a symmetrical pattern, guide signals transmitted through the internal conductor. In addition, the conductive welding portion may be circular. It is possible to guarantee room for other circuits by forming the conductive welding portion in a circular shape; thus, minimizing the space occupied by the conductor.
Preferably, the coaxial cable is vertically coupled with the planar circuit substrate to reduce the loss of signal-and to simplify the manufacturing process, in comparison to the horizontally connected coaxial cable and the planar circuit substrate.
To further solve the above-described problems, another aspect of the present invention is to provide a connection structure, in which an external conductor of a coaxial cable is electrically connected to a ground conductor installed at a surface of a planar circuit substrate and an internal conductor of the coaxial cable is connected to a micro strip line installed at the other surface of the planar circuit substrate, wherein at least one via hole is prepared by a predetermined method in a predetermined pattern in the planar circuit substrate around the internal conductor of the coaxial cable coupled with a portion of the micro strip line by a predetermined method. Here, it is preferable to form at least two via holes in a symmetrical pattern.
According to the present invention, it is possible to improve signal transmission characteristics by forming a via hole in the place where a coaxial cable and a planar circuit board are connected, and guiding signals transmitted through the internal conductor of the coaxial cable. The via hole can be used in a configuration according to the present invention in which no additional connector is required, and in other existing configurations having a connector.
According to the present invention, it is preferable that a conductive pattern of the micro strip line comprises a conductive welding portion welded to the internal conductor of the coaxial cable, a conductive landing portion extending from the conductive welding portion on the same plane as the conductive welding portion, and an impedance converting portion extending from the conductive landing portion with an increasing width.
According to the present invention, it is possible to improve signal transmission characteristics by guiding signals by a via hole and by setting a characteristic impedance with a predetermined value by configuring a micro strip line as described above.